Extended Energy Elemental Spectrum Measurements with the Solar Isotope Spectrometer (SIS)

1. Extended Energy Elemental Spectrum Measurements with the Solar Isotope Spectrometer (SIS) • A. W. Labradora, C. M. S. Cohena, A. C. Cummingsa, R. A. Leskea, R. A. Mewaldta, • E. C. Stonea, T. T. von Rosenvingeb, M. E. Wiedenbeckc • (a) California Institute of Technology, MC 220-47, Pasadena, CA 91125 USA • (b) NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA • (c) Jet Propulsion Laboratory, Pasadena, CA 91109 USA • 29th International Cosmic Ray Conference • Pune, India • 3-11 August 2005

2. Abstract: The Solar Isotope Spectrometer (SIS) aboard the Advanced Composition Explorer (ACE) has been measuring solar energetic particle abundances since ACE was launched in August 1997. SIS is a silicon detector telescope that identifies charged particles via the multiple dE/dx vs. total energy technique, and to date, most measurements reported by SIS have been limited in energy to those particles that stop in the instrument. In this poster, we summarize the multiple dE/dx technique used to identify particles that penetrate through the bottom of SIS, extending the energy ranges for elemental abundance measurements. In preliminary analysis, the upper energy limit for oxygen has been extended from ~90 MeV/nuc for stopping particles to ~300 MeV/nuc for penetrating particles, and the upper energy limit for iron has been extended from ~168 MeV/nuc to ~500 MeV/nuc. We have begun to apply this technique to SIS measurements of large SEP events, such as the Nov. 6, 1997 event and the recent Jan. 20, 2005 event, to extend the measured spectra, to look for evidence of spectral breaks.

3. The SIS instrument is composed of two identical silicon detector stacks, each topped by a pair of position sensitive matrix detectors to provide trajectory information for particles entering the instrument. The matrix detectors are 70-80 µm thick, with 33.9 cm2 active area. Each stack beneath the matrix detectors is composed of 15 high purity silicon detectors, of ~65 cm2 active area and of individual thicknesses varying from 0.1 mm to 1 mm, and the individual detectors are collected in groups of 1 to 6 of the individual silicon detectors, for a total of 8 detector groups per stack. The detector groups are labeled T1 through T8. The instrument is described in detail by E. C. Stone, et al., Space Sci. Rev., 86, 357 (1998).

4. The Solar Isotope Spectrometer (SIS)

5. Stopping and Penetrating Particles in SIS Data

6. Range-energy curves derived from calculations by H. H. Andersen and Z. F. Ziegler, "Hydrogen: Stopping Powers and Ranges in All Elements", Volume 3 of The Stopping and Ranges of Ions in Matter (Pergamon: New York 1977). • The foldback point marks the transition from stopping to penetrating particles. • Stopping particles show both element and and isotope separation (in some cases). • Penetrating particles also show element separation up to regions near minimum ionizing. • Given Z, A, and SIS detector responses, the energy for a given particle is estimated from a best-fit vs. E to the corresponding range-energy curve. Roughly speaking, the energy is a function of the particle’s position along the penetrating portion of the range-energy curve.

7. The 6 November 1997 event included large x-ray emissions followed by large particle fluxes, reaching maximum over several hours, up to a day. It was detected as a ground-level event, although it was not as large as the 20 January 2005 event. The January 20 event was the largest ground level event detected by neutron monitors in decades. It was accompanied by large x-ray emissions, followed within minutes by large particle fluxes with rapid rises in intensity. The particle spectra themselves are notable for their hardness, extending over wide energy ranges, for elements up to and including iron.

8. http://goes.ngdc.noaa.gov/data/plots/1997/

9. http://goes.ngdc.noaa.gov/data/plots/2005/

10. 6 November 1997

11. 20 January 2005

12. A test of the energy-measurement algorithm for penetrating particles can be obtained by treating RNG 7 stopping particles as "penetrating" and ignoring the energy deposited in the T7 and T8 detectors. These figures show energy ranges for RNG 7 and RNG 8 as horizontal, cross-hatched bars, calculated using the penetrating particle algorithm applied to a large sample of quiet time data. The black bar at the top shows the corresponding RNG 7 energy range for stopping-particle analysis. The energy histograms shown are from the 20 January 2005 event, for visual comparison for two relatively abundant elements (N and O) and one sparse element (Si). The figure shows good agreement between RNG 7 energy ranges obtained via the two techniques, demonstrating reasonably acceptable accuracy for the penetrating particle analysis. Results for selected elements (C, N, O, Ne, Mg, Si, and Fe) in a large sample of quiet-time data show the potential to extend the SIS energy range by a factor of ~2.

13. Testing the Penetrating Particle Energy Measurement Technique with Stopping Particles

15. Hard cuts were applied to the penetrating oxygen data in order to reduce contamination of the measurement from adjacent elements, so the oxygen energy ranges extend only as high as ~150 MeV/nuc. Analysis has not been completed to obtain absolute intensities for penetrating particles; penetrating particle intensities are scaled to lie near projections from the stopping particle intensities. Until absolute intensities are available, relative intensities for points within the penetrating particle energy ranges may be used to look for evidence of softening or hardening of spectra relative to the stopping particle spectra. Current results are inconclusive, particularly for the 20 January 2005 event, but the penetrating particle spectra for the 6 November 1997 event fit the power law projections well.